They might be very good, but "very good" is not enough to displace experienced senior professionals.
snip
Why?
Do you deny that even top clubs have sub-elite/rotational/squad players?
If there are "very good" young players, why do top clubs spend 30m+ to get similar level squad players? I'm flipping your argument about Neymar and Messi. They are what they're now because they got opportunities at that stage. Santos gave him the chances, but do you think every player who was given chances at Brazil become elite? Santos gave Robinho similar chances in 2002, when he was 17/18 just like they did to Neymar at such a young age. Did Robinho become elite? It's not an exact science at ages 17-18 for everyone/clubs to know who will and will not become elite and say we won't give chances to those who won't become elite. Messi came through, because Barca were genuinely interested in giving chances to their academy graduates. Pedro and Sergi Roberto also came through Barca, but nobody is calling them elite, right?
If you think it's that generic and every top club follows a similar model with respect to just promoting elite talents and discarding anything below that, then do you think Pedro is an elite player? I don't think so. Yet, he got the opportunities at Barca, and became the player he is now, which is "very good", but not elite, because Barca are a club who're decent at bringing players through the academy. Yes, they spend a lot as well, but they look at academy players from time to time. But how many other Top Clubs do that?
Do you think Mbappe could've come through PSG as easy as he did at Monaco? Yes, now we know he's Mbappe because he was given the opportunity at Monaco, but what if he was not? Do you think PSG would've spashed the cash on him without those chances at Monaco? Or would they have given him the same opportunities like Monaco did if he came through PSG academy?
Money is just not an 'EXCUSE' not to look at your academy. It just says incompetence/disinterest with the respective club's academy. Even Top clubs can bring through squad players instead of giving big cash for squad/non-elite players - see Barca. But most top clubs just don't/can't do it better, and take the easier route with spending because they can afford to. Why are Chelsea buying all those players/loaning them out and then selling them for big money? Just to bring elite talents in? Do you think they believed that all the 40 youngsters they bought and have them out on loan are elite? No. They do that because they can sell better and that they have no clue how to do that with their academy players, so they take the easier route of sending them to other clubs who will give these kids chances & then hope to hit the jackpot with a few. If every club knew how to do the same with their own academy players or had a complete idea about developing young players from talents to elite players, then they will do that, believe me. But development is not linear and they just don't know how to do that is all.